Fluid pumps



A ril 25, 1961 Filed June 26, 1957 L. WEISSMANN ET AL FLUID PUMPS 7 Sheets-Sheet l INVENTORS Le a/1 WE/SSMAN/V LuDGER vamaer .1:

April 1961 L. WEISSMANN ETAL 2,981,199

FLUID PUMPS Filed June 26, 1957 7 Sheets-Sheet 2 INVENTORS Lei; WEISSMA/VN LUDGEK v04 Paw J3 610k LA 1961 L. WEISSMANN ETAL 2,981,199

April 25 FLUID PUMPS 7 Sheets-Sheet 3 Filed June 26, 1957 lNVE/VTORS Leo/7 WE/55MA/VN LUDGEK VOLPE/QT April 25, 1961 w 5 N ETAL 2,981,199

FLUID PUMPS Filed June 26, 1957 7 Sheets-Sheet 4 //V VENTORS Leo/7 WE/SSMA NN L00 GER VOL PER r APril 1961 L. WEISSMANN ETAL 2,981,199

FLUID PUMPS 7 Sheets-Sheet 5 Filed June, 26, 1957 /N VENT 0R5 April 1961 1.. WEISSMANN ETAL 2,981,199

FLUID PUMPS Filed June 26, 1957 7 Sheets-Sheet 6 l/V VENTOR 5 Leon WEISSMANN 6 V LPERT United FLUID PUMPS Leon Weissmann and Ludger Volpert, Dusseldorf, Germany, assignors to Becorit Grubenausbau G.rn.b.I-I., Recklinghausen, Germany Filed June 26, 1957, Ser. No. 668,239

Claims priority, application Germany June 28, 1956 2 Claims. ((31. 103-=-3) The present invention relates to fluid pumps particularly adapted for use in apparatus exerting pushing or pulling forces for example in pit props.

in a known construction of this character, the pressure fluid pump is driven by means of a double-acting compressed air piston motor which is arranged inside the prop. Since a pressure of, for instance, about 500 to 600 atmospheres has to be applied in bracing the prop between the roof and the floor, the compressed air motor has necessarily to have a piston of considerable dimensions to deliver an adequate quantity of fluid at a high pressure of this order during the comparatively slow movement of the piston.

The slow operation of a compressed air motor of this form also calls for a pressure fluid pump of large dimensions, and this involves the disadvantage that the pump and pump drive means occupy a large part of the internal space in the prop in this known form of hydraulic pit prop. The comparatively small rate of reciprocation of the compressed air motor, moreover, means a small delivery output from the compression fluid pump per unit time, as a result of which the setting and bracing of the prop occupies a relatively considerable period. A further drawback of this known form of pit prop lies in the fact that the compressed air cylinder and the pump require a large number of channels, valves, and control mechanisms, and this complicates the structure of the prop and involves continual maintenance.

It is an object of the present invention to eliminate the drawbacks attaching to this known form of construction, to which end a vibratory motor, which can be operated by a low voltage alternating current or a chopped direct current, is used to operate the pressure fluid pump.

As a consequence of the high rate of reciprocation of the vibratory motor, which is equivalent to the frequency of the alternating current or the chopped direct current, the volumetric stroke of the pump cylinder can be kept very small even when the output of the pressure fluid pump is comparatively large. This presents the advantage that a motor of very small driving power is adequate to operate the pump, a voltage of, for example about 24 volts being adequate for directly operating such a motor.

The use of such a low voltage, moreover, affords particular advantages in subterranean mining operations, having regard to the danger of fire damp being present or of electrical short-circuits. Thus, special protection against fire damp or a pressure-tight casing of the motoras are invariably necessary in electric motors of other kinds in underground rnining-can be dispensed with, the development of arcing or shorting being excluded in the case of a vibratory motor. As a result of this, and of the small motive power which is required, the vibratory motor can be made of very small dimensions.

Moreover, since the pump may also be made very small, by reason of the high rate of reciprocation, the pump and its drive means can be arranged without dif- 2,981,199 Patented Apr. 25, 1961 any increase in the dimensions of the latter.

. In an advantageous embodiment of the invention, there is associated with the electromagnet of the vibratory motor at least one armature which is adapted to be attracted against the action of a return bias and the reciprocating movement of the pump piston is derived from the oscillating movements of this armature, which are synchronised with the frequency of the voltage variations. It is then advantageous to make the armature or armatures of the vibratory motor in the form of a lever 0r levers arranged more or less perpendicular to the longitudinal axis of the pump and rockable in the direction of ,movement of the pump piston associated therewith, the pump piston being axially displaced against the action of a spring element by the rocking of the lever or levers towards the vibrating magnet.

This arrangement has the advantage of dispensing with complicated gears and transmission means between the armature or armatures of the vibratory motor and the piston of the pressure fluid pump. The vibratory motor is then only required to exert an effort in one direction of movement of the pump piston, the return stroke being produced by the restoring effort stored up in the spring element in the preceding stroke.

If use is made of a two stage pressure fluid pump, it is advantageous to provide rocking levers at both ends of the vibrating magnet and to couple them with oppositelyrnoving high and low pressure pistons of the pump. Since the opposed movement of the rocking levers can be converted very simply into the opposedmovement of the pump pistons, a double-acting vibratory motor of this character is very suitable for operating a two-stage pressure fluid pump, and a pump of this kind has the advantage that both pressure stages can be used to alternately deliver fluid during the propulsion of the inner prop member outwards until it is applied against the roof, whilst during the ensuing bracing of the prop between the roof and the floor only the high pressure stage operates.

The arrangement, in this case, is advantageously such that the high pressure piston is co-axially arranged and guided in the low pressure piston which then takes the form of a high pressure cylinder. In this event it is of advantage to make the coupling to the vibratorymotor such that, the high pressure piston is driven by the rocking levers against the action of the compression spring means during its compression stroke whilst the low pressure piston is operated by these rocking levers against the spring action during its suction stroke.

After the setting and bracing of the prop, the vibratory motor is advantageously actuated by a pressure relay which is governed by the fluid pressure in the compression chamber of the prop, this relay being adapted to automatically cut out the vibratory motor after a suitable, adjustable fluid pressure has been reached, and should any reduction in the fluid pressure then occur for example as a result of imperfections in the sealing of the pressure cylinder, to re-start the vibratory motor to restore the pressure.

Again, a pressure relief valve associated to the compression chamber can be opened electrically by a pressure relay controlled by the fluid pressure. Similarly the return flow valve, which is to be opened during the robbing or collapsing of the prop, can be electrically operated by an electromagnet.

The invention is illustrated by way of example in the accompanying drawings, in which:

Figure 1 is a longitudinal section through the lower part of a hydraulic pit prop according to this invention with its pump and pump driving means,

Figure 2 illustrates the part of the prop seen in Figure l but with the pump piston in a different position,

Figure 3 is a section on the line lII-III of Figure 1,

Figure 4 is a section on the line IY-IV of Figure 3,

Figure 5 is a longitudinal section through the lower half of another embodiment of the hydraulic pit prop, involving a different arrangement of the pump and pump driving means,

Figure 6 is a diagram showing the electrical connections of a vibratory motor in accordance with this invention, I

Figure 7 is a longitudinal section through the complete prop according to Figures 1 and 2, the pump and its driving means being indicated diagrammatically.

Figure 8 is a longitudinal sectionthrough the complete prop according to Figure 5, likewise with the pump and the pump drivingmeans shown diagrammatically.

The hydraulic pit prop illustrated in Figures 1 to 4 and 7 comprises a tubular outer prop member l having longitudinally displaceable, and guided in sealed fashion, therein an inner prop member 2 which is likewise of tubular form. The inner prop member 2 is closed at itslower end by a piston 3. The inner chamber 2a of the inner prop member 2, which is closed in fluid-tight fashion against the outer atmosphere, serves as a supply container for the pressure fluid, preferably oil. The section 1a of the length of the outer prop member 1 located below the inner prop piston 3 serves as a compression cylinder.

Provided in the piston 3 of the inner prop member chamber 1a of the prop through a relatively constricted nozzle 6. The walls of nozzle 6 at the low pressure cylinder side are flared at an angle of about 120 and at the compression chamber side are flared at an angle B of about 12 to 16. The low pressure cylinder 4 is open at its end adjacent the inner chamber 2a of the inner prop member 2.

Passing co-axially through the low pressure piston 5 is a cylindrical bore 7 which is closed at the end thereof adjoining the low pressure cylinder 4 by a return flow valve 8. The valve body 8a of this valve is held in the closed position by a compression spring 9.

Guided for longitudinal and sealed movement in the cylindrical bore 7 of the low pressure piston 5, which for example is of brass or mild steel, is a high pressure piston 10 of case hardening steel. Secured to the upper end of the high pressure piston 10 is a head 11 which is of disc form and serves as a bearing for a compression spring 12 which has its other end applied against a bearing 13 on the low pressure piston 5. The compression spring 12 surrounds the high pressure piston 1.0 and a guide sleeve M which adjoins the bearing 13 on the low pressure piston.

The pistons 5 and 10 of the pressure fluid pump formed by the parts of the prop described above are actuated, so as to move counter to one another as later explained, by means of a vibratory motor 15 which is operated by an alternating voltage of 24 volts at a frequency of 50 Hertz. The vibratory motor 15 consists of a magnetic winding 16 which is fed with alternating current and has an elongated rectangular shape, as illustrated in cross section in Figure 3. The winding 16 surrounds a magnetic core 17 composed of laminated iron plates. Further sets of laminations 17a of sheet iron are arranged at the outer sides of the winding 16 to improve the magnetic flux.

The electromagnet constituted by the parts 16, 17, 17a is disposed substantially parallel to, and is spaced above the face of the piston 3 nearest the supply container 2a, and is rigidly connected to this piston 13. Armatures 18, 18a of laminated iron plates are disposed symmetrically on either side of the electromagnet 16, 17, 17a. The armatures 18, 18a are fixed to a carrier arm 20, rigidly connected to the inner prop piston 3, by means of a spring clip 19, 19a. The free end of the upper armature 18 bears against the rear face of the head 11 of the high pressure piston 10. The free end of the lower armature 18a engages face of the bearing 13 on the low pressure cylinder 5 opposite to that against which the spring 12 is applied.

The armatures 18, 180! which are arranged substantially at right angles to the longitudinal axes of the pump pistons 5, 10 and act as rocking levers, can perform a rocking motion in the direction of movement of the pump pistons 5, 10 by virtue of their resilient mounting. Figure 1 illustrates the outer limit position of the rocking levers 18, 13a, in which these are held by the spring action of the compression spring 12 and of the spring clip 19, 1%, the vibratory motor 15 being inoperative.

The electromagnetic winding 16 of the vibratory motor 15 may be selectively connected to direct current source or to an electric alternating source at 24 volts, for example by means of the switch connections illustrated in more detail in Figure 6. Should this winding be connected to the direct current source, for example to a rectifier, the rocking levers 18, 18a are attracted by the magnetic flux of the electromagnet 16, 17, 17a and rocked into abutment with the end face of this electromagnet, as illustrated in Figure 2. When the rocking levers 18, 18a move in this way they shift the high pressure and low pressure pistons, 10 and 5 respectively, of the fluid pump in convergent fashion against the restoring bias of the compression spring 12 and the spring clip 19, 19a.

When the direct current supply to the winding 16' of the vibratory motor is interrupted, the levers 18, 13a

rock under the action of the compression spring 12 and winding sets up an alternating magnetic field which first attracts the rocking levers 18, 18a against the return bias of the springs 12, 19, 19a until they abut against the end faces of the electro magnet 16, 17, 17a, and then releases them again so that these levers are swung back by the springs 12, 19, 19a into their outer limit positions, and so on in alternating rhythm with the current supply.

At an alternating voltage frequency of 50 Hertz, which is admirable for operating the vibratory motor, the rocking levers 18, 18a make 100 to and from strokes per second, so that the rate of reciprocation of both pump pistons is likewise 100 per second. This high rate of reciprocation enables the cross sectional dimensions and lengths of stroke in the pump cylinder to be kept very small. Thus, for instance, the cross section of the low pressure piston 5 may be of 2 cm. and the cross section of the high pressure cylinder 7, for example, 7 mm With a stroke of say, 0.5 cm. the low pressure stage delivers 0.1 litre pressure fluid per second when operating at 100 strokes per second. This rate of fluid delivery is completely adequate for setting the prop, during which action the pressure fluid is delivered by the low pressure piston 5 into the compression chamber 1a of the prop, having regard to the fact that about 2 to 3 litres of pressure fluid, which amount is required for setting the prop, can be delivered to the compression chamber 1a of the prop in 20 to 30 seconds with the dimensions quoted above. It is then suflicient for an overpressure of 0.1

atmosphere to be developed in. the low pressure cylinder 5 with 'a-pistonsurface'of'l cmF, 'a'forceof 0.2 kg. only is necessary. The compression spring 12 required to apply this force can therefore be made of very small dimensions.

It the piston face of the high pressure piston is 7 mm. and this piston has a stroke of 5 mm., this piston will deliver 3.5 cm. fluid per second. When the inner prop member is driven outwardly until it bears against the roof, the high pressure piston merely has to overcome the closing effort of the spring loading the valve 8. The high pressure piston 10, during the upward movement produced by the action of the spring 12, draws fluid from the supply container 2a through overflow channels 21 in the rear part of the high pressure cylinder 7. During the downward movement of the high pressure piston 10 under the action of the electromagnet 16, 17, 17a, the pressure fluid trapped in the high pressure cylinder 7 is compressed by this piston 10, once the ports of the suction channels 21 have been closed, opens the valve 8, and is delivered into the low pressure cylinder 4.

Since the quantity of pressure fluid entering the low pressure cylinder 4 in this way is not adequate to fill this cylinder 4 during the suction stroke of the. low pressure piston 5, a spring-loaded suction valve 22, illustrated in section in Figure 4, is connected in parallel with the high pressure cylinder.

The suction valve 22, see Figure 4, controls, in fact, an overflow channel 23, 23a which connects the lower end part of the low pressure cylinder 4 to the supply container 2a. During the suction stroke of the low pressure cylinder 5 this draws pressure fluid from the supply container 2a of the prop through the return flow suction valve 22 and into the low pressure cylinder 4 in addition to the quantity of fluid which is transferred from the high pressure cylinder 7.

As long as the pressure fluid in the compression chamber In of the prop does not exceed the pressure which is required to push out the inner prop member until it bears against the roof, both pressure stages of the fluid pump operate when the vibratory motor is coupled to the alternating voltage, the high pressure stage, however, merely delivering into the low pressure cylinder 4. Having regard to the small pressure differential between the low pressure cylinder 4 and the compression chamber 1a of the prop, which is of the order of about 0.1 atmosphere, and the high reciprocating speed of 100 strokes per second, the nozzle 6 merely allows the pressure fluid to pass from the low pressure cylinder 4 into the compression chamber In of the prop during the pressure stroke of the low pressure cylinder 5. In the opposite direction, the nozzle 6 offers a substantially greater resistance to the fluid flow, so that at the high rate of reciprocation and the small pressure differential, the pressure fluid cannot, during the suction stroke of the low pressure piston 5, flow back from the compression chamber 1a into the low presure cylinder 4., The nozzle 6 thus acts, during the operation of the low pressure stage of the pressure fluid pump, as a non-return valve which opens only into the compression chamber 1:: of the prop.

As soon as the head of the inner prop member 2 abuts against the roof, the increasing fluid pressure in the compression chamber 1a is propagated through the nozzle 6 into the low pressure cylinder 4. The result is that the low pressure piston 5 is arrested in its upper limit position illustrated in Figure 2, the pressure of the compression spring being insufiicient to overcome the fluid pressure operating on the low pressure piston 5. As a result the low pressure stage in the operation of the pressure fluid pump is cut out automatically when the inner prop member has been pushed out sufliciently to meet the roof resistance. On the other hand, the high pressure piston 10 which is moved during the thrust stroke by the attraction of the electromagnet 16, 17, 17a, continues until the full operating presure is developed in the pressure chamber 1a of the prop.

Where the piston face of the high pressure piston 16' is 7 mm. and the fluid pressure in the compression chamber 1a of the prop is 500 atmospheres, a force of 35 kg. has to be applied by the. electromagnet 16, 17, 17a during the thrust stroke of the high pressure stage. For the suction stroke of the high pressure piston 10, the spring effort of the spring 12, amounting for example to 200 gr., is adequate.

As can be seen from Figure 2 the valve body 8a of the pressure valve 8 of the high pressure stage is equipped with an extension which is guided with play in the high pressure cylinder 7. When the pump pistons are in the inner limit position, this extension lifts the valve body 8a from its seat. The valve 8 is therefore opened immediately the vibratory motor 15 is switched over to direct current. The pressure fluid can pass, after opening the valve 8, through a return flow channel 24 provided in the high pressure piston 10 into the overflow channels 21 connected to the supply container 2a and consequently flow back from the compression chamber 1a into this supply container. The overflow channel, which consists of a longitudinal bore and at least one transverse bore in the high pressure piston 10- is so arranged that it is connected to the overflow channels 21 when the pump piston 5 is at its inner limit position only. The passage of pressure fluid from the compression chamber 1a to the low pressure cylinder 4 through the nozzle 6 is not obstructed when the pump is stopped.

A second bore 25 of large free cross sectional area is arranged in the piston 3 parallel to, and at a distance from, the cylindrical bore 4 passing through this piston, the bore 25 being provided in a sleeve 29 and closed by a return flow valve which can be opened in direction to the compression chamber 1a of the prop, in order to allow the pressure fluid to flow back to the supplying container 20. The valve body 26a of valve 26 is held in the closure position under-the fluid pressure obtaining in the compression chamber 1a.

A magnetic armature 27 is rigidly connected to the valve body 26a by means of an extension 261) which is guided with play in the bore 25. The sleeve 29 is in the form of a cylindrical iron core and is surrounded by a direct current magnetic winding 28 having a casing 29a of ferro-magnetic material. Other than the iron core 29 and the casing 29a of the winding 28, the inner prop piston 3 is of non-magnetisable material, for example aluminium or a synthetic substance. This prevents the magnetic flux produced by the electromagnet 28, 29, 29a being distorted by the inner prop piston 3.

When the winding 28 is connected up to a direct current source, the armature 27 of the electromagnet 28, 29 is attracted. The force of the attraction is only sufficient, however, to lift the valve body 26a of the valve 26 from its seat when the pressure is low in the compression chamber 1a. The valve 26, therefore, can only open when there has been a considerable exhaust of pressure from the compression chamber 1a of the prop as a result of opening of the high pressure valve 8.

An advantageous arrangement of the connections of theelectromagnetic winding 16 of the vibratory motor 15 and the electro-magnetic winding 28 of the valve 26 is illustrated in Figure 6. The winding 16 can be connected selectively to alternating current contacts 31, 31a, or to direct current contacts 32, 32a by means of a double-armed switch 30. Mains voltage stepped down to 24 volts serves as the alternating current, whilst the di- "rect current is taken from a rectifier 33 which is advanup to a direct voltage source through the intermediary.

ot a chopper of conventional construction, and the direct amines current contacts 32, 32a are directly connected to this source.

A chopper direct voltage can also, of course, be produced at the alternating current contacts 31, 32. by connecting these in known fashion through alternating-current, half-wave rectifiers to a source of alternating current.

1 In addition to the manually-operated switch (not shown) a control relay 34 is provided for controlling the vibratory motor 15, this relay being operatedby the fluid pressure in the compression chamber of the prop against the restoring bias of a spring. When the prop is being set, the double switch 30 is connected manually to the alternating current contacts 31, 31a, the prop being extended and braced between the roof and floor. When the operating pressure has been built up in the compression chamber 1a of the prop, the double switch 30 is rocked in the direction x by the piston rod 34a of the control piston 34 and separated from the contacts 31, 31a. This automatically cuts out the vibratory motor 15.

iii

Should the fluid pressure in the compression chamber 1a exceed the maximum permissible pressure, the double switch 30, and thus the alternating current electromagnet 16,17, 170, are connected up by the control relay 34 to the direct current contacts 32, 32a. As a result, as illustrated in Figure 2, the pressure relief valve 8 is opened so that the pressure fluid can flow back from the compression chamber 1a into the supply container 2a. If the pressure in the compression chamber In drops again to the operating pressure, the double switch 3%, which is urged in the direction x by a spring not shown in Figure 6, is separated from the contacts 32, 32a and the valve 8 thereby closed.

Should the pressure in the compression chamber 1a fall, for example as a result of improper sealing, below the operating pressure, the control piston 34 is adapted to be shifted, by the spring which has been loaded thereby, sufiiciently for the double switch 30 to pivot, under the action of the spring element which urges it in the direction x until engagement with the alternating current contacts 31, 31a is re-established. As a consequence, the pressure fluid pump again becomes operative and continues to run until the pressure relay 34 separates the double switch 30 from the alternating current contacts 31, 31a, the operating pressure having been reached. The operating pressure and the maximum permissible pressure of the prop can be adjusted very simply by variation of the spring loading of the spring element blessing the control piston 34, or by variation of the spacing of the contacts 31, 31a and 32, 32a.

To rob or collapse the prop, a bell crank lever 36, pivotable about the point 35, is manually rocked in the direction y. The contact 32, which is secured to the longer lever arm of the lever 36, is then brought into engagement with a contact 37, whilst the shorter arm of the lever 36 pivots the double switch 34 in the direction x, independently of the control relay 34. Engagement between the contacts 32 and 37 connects the winding 23 of the valve 26 in series with the winding 16 of the vibratory motor. The resistance of the direct current magnet 28 prevents any undesirable heating up of the alternating current magnet 16 of the vibratory motor, even if the direct current circuit remains closed over a long period. With the double switch in this position, the high pressure valve 8 is first opened by the electromagnet 16, 17, 17a of the vibratory motor, which magnet has direct current flowing therethrough, and a pressure balance established between the compression chamber 1a and the supply container 2a. Following on this the return flow valve 26, which has a substantially larger flow cross-sectional area than the valve 8, is opened by the winding 28 of the valve 25, so that the pressure fluid can flow back rapidly from the compression' "chamber 121 into the supply container 2a. "lnFigures 7 and 8the supply container 2a is equipped at its upper 'end with two air pressure balancing valves. It is however also possible, well to seal the supply container 2:; from atmosphere. In this case a vacuum is set up therein when the prop is extended, and this vaccum accelerates the return flow of the pressure fluid once the valve 26 has been opened, and provides for a rapid collapsing of the prop.

In the embodiment of the invention illustrated in Figures 1, 2 and 7, all the movable parts of the vibratory motor 15 and the pressure fluid pump 5, 10 are surrounded by the pressure medium, preferably oil, this providing a continuous lubrication thereof.

The direct and alternating currents are advantageously fed to the individual props by a powerful, and preferably armoured, four-strang cable which is used as a tow cable when the prop is collapsed.

If use is made of fluid pumps with a plurality of stages, it is obviously possible to provide a special vibratory motor for operating each pump piston. If no alternating current supply is available, the vibratory motors may be coupled up to a direct current source, through a chopper.

On account of their small dimensions, the pressure fluid pump and the vibratory motor can be arranged in any appropriate or desired position inside or externally of the prop, other than the arrangement illustrated in Figures 1 to 4 and 7. Thus it is feasible, for example, to make the pump and pump driving means as separate units which are only secured tothe outside of the prop immediately before the latter is to be set and are then connected up in a pressure fluid line leading to the compression chamber of the prop. Further, a common pressure fluid pump operated by a vibratory motor can be provided for a plurality of props, this pump being connected to these props only when they are to be set. In this event it is required, of course, to equip .the compression chamber of each prop with a pressure relief valve and a manually-adjustable pressure-reducing valve.

Figures 5 and 8 illustrate a further embodiment of the hydraulic pit prop according to the invention in which the pressure fluid pump and the associated means for driving it are located in the bottom of the outer drop member 1.

In this prop construction the end of the inner prop member 2 adjacent the pressure chamber 1a of the outer prop member 1 is closed by an annular piston 3a. Passing through the piston 3a is a pressure-balancing cylinder 38 which is co-axial with the prop and is of an axial length corresponding to the range of adjustment of the two prop halves 1, 2. The balancing cylinder 38 is open at both ends and is in permanent flow communication with the supply container 2a. The end of the balancing cylinder 38 nearest the bottom of the compression chamber 1a has an outwardly-directed flange 39 which is forced by the fluid pressure in this chamber against the bottom of the latter. The annular piston 3a is longitudinally displaceable in sealed fashion between the inner wall of the chamber In and the outer wall of the cylinder 38.

In the embodiment illustrated in Figures 5 and 8, the alternating current magnet 16, 17, 17a of the vibratory motor is of annular formation and secured in a cylindrical recess 40 in the floor or base of the compression chamber. The winding 16 is disposed between an annular iron core 17 and an annular casing 17a of laminated iron plates. The armatures 18, 18a associated with the magnet 16, 17, 17a are of disc form and rigidly connected to the high pressure piston 10 and the low pressure piston 5 respectively. The armatures 18, 18a are biassed in opposite directions by the high pressure piston 10 and a compression spring 12 sun'ounding a guide sleeve 14 provided at the rear end of the low pressure piston 5.

The low pressure piston 5, which is axially displaceable in the low pressure cylinder 4, is provided with, an annular groove 5a which is connected to the high pressure cylinder 7 through a plurality of-radial ducts 5b. The

9 annular groove a and the ducts 5b are so arranged that the high pressure piston is able to draw compression fluid, during the suction stroke, through a channel 41, which opens into the inner chamber of the balancing cylinder 38, whilst the openings from the ducts 5b into the cylinder 7 are closed by the high pressure piston 10 during the pressure stroke, so that the pressure fluid is compressed and forced through the spring loaded valve 8 into the upper cylinder chamber of the low pressure cylinder 4. This part of the low pressure cylinder 4 is connected through a pressure conduit 42 to the compression chamber 1a of the prop.

As can be seen from Figure 5, the part of the pressure conduit 42 opening into the compression chamber In is, similarly to the embodiment illustrated in Figures 1 to 4, in the form of a nozzle 6 of relatively small flow cross section with the partof the Wall thereof which opens into the compression chamber of the prop flared at an angle of about 12 to 16 degrees to the axis of the nozzle. During the operation of the low pressure piston 5, this nozzle operates, similarly to the embodiment described in relation to Figures 1 to 4, as a valve which prevents a return flow of the pressure fluid from the compression chamber in into the low pressure cylinder 4. The upper cylinder chamber of the low pressure cylinder 4 is connected to the inner chamber of the balancing cylinder 38a by a spring-loaded suction valve -43.

A direct current magnet 28, 29 is arranged laterally of the pressure fluid pump at the bottom of the outer prop member 1 and has a pivotally-mounted armature 44 associated therewith. When no current flows through the magnet 28, 29, the armature 44 is held in the position illustrated in Figure 5 under gravity.

When the magnetic winding 28 is connected to a source of direct'current, the armature 44 is attracted and lifts the cylinder 38 from its seat through a thrust bar 45. The balancing cylinder 38, which is applied forcibly against its seat by the fluid pressure in the compression chamber in, can, however, only lift upwardly in the manner described above when a pressure balance exists between the compression chamber 1a and the supply container 2a. This pressure balance is brought about by the fact that, when the vibratory magnet 16, 17, 17a is switched over to direct ctu'rent, the end face of the high pressure cylinder 10 opens the valve 8 thus enabling the pressure fluid to flow back from the compression chamber 1a into the interior of the cylinder 38 through the conduit 42, the low pressure cylinder 4, the high pressure cylinder 7, the ducts 5, the annular groove 5a and the channel 41. The compression chamber 1a is directly connected to the inner chamber of the balancing cylinder 38a by the ensuing raising of the cylinder 38 from its seat, so that the pres- 10 sure fluid can flow very rapidly out of the compression chamber 1a and there is a rapid telescoping of the two halves of the prop.

We claim:

1. In a two-stage pressure fluid pump particularly for fluid pressure actuated apparatus such as pit props, the improvement comprising in combination a cylinder; a closure member on one end of said cylinder; a high pressure piston reciprocable in said cylinder; said closure member and said high pressure piston defining a high pressure chamber; a cylindrical boring in said high pressure piston extending at least substantially in the direction of the axis of said cylinder; a low pressure piston slidably mounted in said boring for reciprocating movement therein; an electric vibratory motor actuating said low pressure piston; a control valve in said high pressure piston connecting said boring andsaid high pressure chamber; said control valve comprising a nozzle of small efiective flow section whereby, when the low pressure piston is operated at a high rate of reciprocation and a small pressure difierential arises between the two sides of the nozzle, the latter allows pressure fluid to flow only to said pressure chamber whilst, in the event of a large pressure differential between the two sides of the nozzle and when the pump is arrested, the nozzle allows pressure fluid to flow in either direction.

2. A two-stage pressure fluid pump as defined in claim 1 wherein said nozzle is composed of two frusto-conical sections connected by a restricted waist, the section facing the low pressure piston having a cone angle of approximately and the other section adjacent the pressure chamber having a cone angle of 12 to 16.

References Cited in the file of this patent UNITED STATES PATENTS 1,364,882 Koken Jan. 11, 1921 1,953,606 Hobson Apr. 3, 1934 r 1,969,920 Andrews Aug. 14, 1934 2,134,501 Bennett Oct. 23, 1938 2,435,326 Schwerin Feb. 3, 1948 2,554,127 Simmons May 22, 1951 7 2,605,613 Grebe Aug. 5, 1952 2,621,631 Dowty Dec. 16, 1952 2,669,186 Parker Feb. 16, 1954 2,685,838 Weinfurt Aug. 10, 1954 2,713,773 Sutton July 26, 1955 2,751,753 Ray June 26, 1956 2,833,221 Dickey May 6, 1958 FOREIGN PATENTS 1,082,903 France June 23, 1954 

